YSK: Use ‘a’ when the word starts with a consonant sound, and ‘an’ when it starts with a vowel sound
Why you should know: The ‘a’ vs ‘an’ conundrum is not about what letter actually begins the word, but instead about how the sound of the word starts.
For example, the ‘h’ in ‘hour’ is silent, so you would say ‘an hour’ and not ‘a hour’. A trickier example is Ukraine: because the ‘U’ is pronounced as ‘You’, and in this case the ‘y’ is a consonant, you would say “a Ukraine” and not “an Ukraine”.
Some modern English words have changed because the leading "n" from the noun migrated over to the article which precedes it, or from the article to the noun.
"Apron" was originally napron, "a napron". "Nickname" was originally ekename (with the first part coming from the same root as "eke", as in "eke out a living"). "An ekename" became "a nekename" and then "a nickname".
This is also true for initialisms, which are acronyms in which each letter is pronounced individually.
"A NASA project" would not become "an NASA project" because nobody pronounces each individual letter of NASA, they just say it as one word.
"An FBI agent" would always be correct, and "a FBI agent" would always be incorrect, because FBI is never pronounced as a word, and each letter is pronounced individually.
The vowel sound rule (or a related one) is also used for which vowel sound goes at the end of the definite article "the", that is, the sound the 'e' makes.
Usually the last vowel sound of "the" is a schwa, arguably the most common vowel sound in English, but before another vowel sound, it becomes "ee", or what other European languages might write "i".
There might even be an intrusive y (or j as used in Norse and Germanic languages) depending on the speaker. i.e. "The apple" may well be pronounced "thi(y)apple", and a fellow native speaker wouldn't notice. "The ball" has the usual schwa. As does "the usual schwa" for that matter.
You can't use any article in front of Ukraine. Not even "the". Just like it's "India", not a/an/the India. It is Britain, but it is also The United Kingdom. For India, you can use The Republic of India.
A good example for your case can be union. It is a union, not an union, because union starts with the sound yu.
I guess I never heard the accents that produced "istoric" in reference to the false americanized version of "an Historic event" such as any time Robert Picard (Richard Woolsey) appeared in Stargate
I'm curious on what others' thoughts are on this: do you say/write "a history" or "an history"? I personally use "a history", but i've seen significant usage of "an history". Do people not pronounce the 'h' in "history"?
TLDW: English speakers increasingly use the consonant versions of "a(n)", "the" and "to" for anything in casual conversation, just with a glottal stop to separate vowel sounds. This is then found more and more in written and formal language.
I think the difficulty people have is when writing English down. In speech they will generally get this stuff right automatically, but when it's on paper "a history honour" can easily look right even though it's not.